| Project Team | Institution | Role | ||
| Assoc Professor Carlos Cáceres | The University of Queensland | Chief Investigator | ||
| Dr Anumalasetty Nagasekhar | The University of Queensland | Research Fellow | ||
| Ms Kun Yang | The University of Queensland | Postgraduate Student | ||
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Executive Summary
New developments: A number of significant events have facilitated the development of the project: the commissioning of an automated microhardness tester, the incorporation of a new postgraduate student, and having access to a new FIB facility in Brisbane.
Two new collaborations have been established: one with Dr Roger Lumley (CSIRO), and one with Dr Philip Eisenlohr, from the Max Plank Institute. With Dr Lumley the joint work aims at characterizing the skin effect in hpdc Al alloys; Dr Eisenlohr is interested in determining the effect of the 3D intermetallic network on the creep properties of their thixomolded MRI 230D alloy. These joint works add to the existing collaboration with Dr Mark Easton (CAST) on RE Mg alloys. Our team will perform SEM/FIB and microhardness mapping characterization in all of these alloys.
In addition, a meeting with Dr Luming Shen (The University of Sydney, project B1.4) and his team was held in order to organise a joint program of experiments and modelling.
Experimental work: The casting and tensile testing of high pressure die cast (hpdc) Mg-Al binary alloys with solute contents of 0.5, 4, 9, and 12 mass%Al has been completed. Testing was carried out on all alloys and for different cross-sectional shapes (rectangular and circular), and different section thicknesses (1, 2, 5, and 10mm).
Microhardness mapping and a detailed micro structural characterization in terms of grain size and distribution has been carried out on the Mg-9mass%Al and Mg-12mass%Al alloys. The data have been used to rationalise the effect of cross-sectional geometry and section thickness on the skin formation. Partial modelling of the strengthening mechanisms has been carried out.
Two and three dimensional micro structural characterisation of Mg-RE binary alloys (Mg-2.87Ce, Mg-3.44La, and Mg-3.53Nd) has been carried out by using SEM and dual beam FIB, respectively. Similar work has been done on R. Lumley's Al-based alloys. The FIB data have to be reconstructed.
New developments: A number of significant events have facilitated the development of the project: the commissioning of an automated microhardness tester, the incorporation of a new postgraduate student, and having access to a new FIB facility in Brisbane.
Two new collaborations have been established: one with Dr Roger Lumley (CSIRO), and one with Dr Philip Eisenlohr, from the Max Plank Institute. With Dr Lumley the joint work aims at characterizing the skin effect in hpdc Al alloys; Dr Eisenlohr is interested in determining the effect of the 3D intermetallic network on the creep properties of their thixomolded MRI 230D alloy. These joint works add to the existing collaboration with Dr Mark Easton (CAST) on RE Mg alloys. Our team will perform SEM/FIB and microhardness mapping characterization in all of these alloys.
In addition, a meeting with Dr Luming Shen (The University of Sydney, project B1.4) and his team was held in order to organise a joint program of experiments and modelling.
Experimental work: The casting and tensile testing of high pressure die cast (hpdc) Mg-Al binary alloys with solute contents of 0.5, 4, 9, and 12 mass%Al has been completed. Testing was carried out on all alloys and for different cross-sectional shapes (rectangular and circular), and different section thicknesses (1, 2, 5, and 10mm).
Microhardness mapping and a detailed micro structural characterization in terms of grain size and distribution has been carried out on the Mg-9mass%Al and Mg-12mass%Al alloys. The data have been used to rationalise the effect of cross-sectional geometry and section thickness on the skin formation. Partial modelling of the strengthening mechanisms has been carried out.
Two and three dimensional micro structural characterisation of Mg-RE binary alloys (Mg-2.87Ce, Mg-3.44La, and Mg-3.53Nd) has been carried out by using SEM and dual beam FIB, respectively. Similar work has been done on R. Lumley's Al-based alloys. The FIB data have to be reconstructed.
Project Aims/Targets
The yield strength of hpdc Mg-Al alloys seems to be determined to a very large extent by the percolating β-phase intermetallic structure (see Figure B9 below) which is thought to have a scaffold-like structure. SEM 2D analysis is not adequate to characterize concave percolating features such as those formed by a percolating intergranular intermetallic. Thus, this study will use on a first stage a sequential sectioning technique to determine the 3D morphology of the intermetallic structure in alloys with predetermined contents of Al in order to characterise the extent of the interconnection.
In situtensile testing will also be used to correlate the plastic behaviour of the alloys with the cracking of the intermetallic structure. This information should enable understanding the origin of the yield strength, the skin effect, the yielding behaviour, the strain hardening behaviour at low strains, the development of damage and the bound imposed by the cracking of the intermetallic structure to the alloy's ductility. A systematic study of a range of alloys with increasing contents of Al is expected to enable isolating and quantifying the contribution to the strength of the grain size, solid solution and dispersion hardening, in addition to the strengthening effects of the 3D network intermetallic microstructure. In the long run, the study is expected to help identify which eutectics produce more efficient reinforcements through the formation of percolating intermetallic structures.
The yield strength of hpdc Mg-Al alloys seems to be determined to a very large extent by the percolating β-phase intermetallic structure (see Figure B9 below) which is thought to have a scaffold-like structure. SEM 2D analysis is not adequate to characterize concave percolating features such as those formed by a percolating intergranular intermetallic. Thus, this study will use on a first stage a sequential sectioning technique to determine the 3D morphology of the intermetallic structure in alloys with predetermined contents of Al in order to characterise the extent of the interconnection.
In situtensile testing will also be used to correlate the plastic behaviour of the alloys with the cracking of the intermetallic structure. This information should enable understanding the origin of the yield strength, the skin effect, the yielding behaviour, the strain hardening behaviour at low strains, the development of damage and the bound imposed by the cracking of the intermetallic structure to the alloy's ductility. A systematic study of a range of alloys with increasing contents of Al is expected to enable isolating and quantifying the contribution to the strength of the grain size, solid solution and dispersion hardening, in addition to the strengthening effects of the 3D network intermetallic microstructure. In the long run, the study is expected to help identify which eutectics produce more efficient reinforcements through the formation of percolating intermetallic structures.
New developments: A series of events allowed for rapid progress on the production of model alloys and on the collection of data. Some very exciting possibilities of collaboration have appeared as well, which have widened the range of alloys to be studied to include other Mg alloys as well as Al alloys. Comparing a range of alloys is expected to furnish information on the ability of different eutectic systems to generate interconnected networks as well as to assess how efficient these structures are to strengthen the material.
Mg-Al model alloys: The team produced a series of model Mg-Al binary alloys, cast in different configurations, which will allow for the testing of a number of crucial hypotheses, and for the completion of the maps of grains sizes and intermetallic microstructures across the sections. Most of these alloys have already been tensile tested; part of the microhardness mapping is to be carried out. SEM analysis is also to be completed.
Al and RE alloys: Two hpdc Al alloys supplied by Dr Lumley, and three RE alloys supplied by Dr Easton, cast in different shapes, have been the subject of SEM/FIB and micro hardness mapping studies to characterize the grain and intermetallics microstructure and the skin effect. 3D reconstruction of the dual beam FIB data is currently under way. A preliminary analysis indicates that the intermetallics are much more interconnected in one of the RE alloys (Mg-3.44La alloy) than in the other two.
Analytical work: A first principles model involving Hall-Petch data, solid solution and dispersion hardening has been produced. It enables to assess (by default) to what extent the interconnected network influences the strength of the material. A simplified version of the model was presented at ICSMA-15 in Dresden, and the full version is currently being written up.
The EBSD data collected on the concentrated binary alloys completed the description we already had on the grain distributions in the dilute alloys. It allowed to develop a comprehensive description of the grain size distribution across the cross section of the castings for the full range of compositions available ( 0.5 to 12mass% Al); It also showed that the grain size near the surface of hpdc's follows the solute growth restriction factor as it would in a quiescent casting. The data also showed that the bimodality in grain distribution is maximised for the alloy with 6% Al (i.e., alloys with composition similar to that of alloy AM60), due to the larger proportion of externally solidified grains (ESG's) in the grain microstructure. The possible effect of ESG's on the deformation behaviour is under analysis using the approach developed by Kurzydlowski and Bucki (Acta Metall. Mater. 41, (1993) 3141-3146).
Collaborations: In a meeting with Dr Luming Shen and his team, the current 3D network structures work was discussed, and it was agreed to collaborate on the study of the local plastic behaviour of the hpdc Mg alloys using the instrumented microhardness tester available at The University of Sydney, and to incorporate the existing 3D files of the intermetallic microstructure into an FE code to model the material's plastic behaviour.
Highlights of results published:
Mg-Al model alloys: The team produced a series of model Mg-Al binary alloys, cast in different configurations, which will allow for the testing of a number of crucial hypotheses, and for the completion of the maps of grains sizes and intermetallic microstructures across the sections. Most of these alloys have already been tensile tested; part of the microhardness mapping is to be carried out. SEM analysis is also to be completed.
Al and RE alloys: Two hpdc Al alloys supplied by Dr Lumley, and three RE alloys supplied by Dr Easton, cast in different shapes, have been the subject of SEM/FIB and micro hardness mapping studies to characterize the grain and intermetallics microstructure and the skin effect. 3D reconstruction of the dual beam FIB data is currently under way. A preliminary analysis indicates that the intermetallics are much more interconnected in one of the RE alloys (Mg-3.44La alloy) than in the other two.
Analytical work: A first principles model involving Hall-Petch data, solid solution and dispersion hardening has been produced. It enables to assess (by default) to what extent the interconnected network influences the strength of the material. A simplified version of the model was presented at ICSMA-15 in Dresden, and the full version is currently being written up.
The EBSD data collected on the concentrated binary alloys completed the description we already had on the grain distributions in the dilute alloys. It allowed to develop a comprehensive description of the grain size distribution across the cross section of the castings for the full range of compositions available ( 0.5 to 12mass% Al); It also showed that the grain size near the surface of hpdc's follows the solute growth restriction factor as it would in a quiescent casting. The data also showed that the bimodality in grain distribution is maximised for the alloy with 6% Al (i.e., alloys with composition similar to that of alloy AM60), due to the larger proportion of externally solidified grains (ESG's) in the grain microstructure. The possible effect of ESG's on the deformation behaviour is under analysis using the approach developed by Kurzydlowski and Bucki (Acta Metall. Mater. 41, (1993) 3141-3146).
Collaborations: In a meeting with Dr Luming Shen and his team, the current 3D network structures work was discussed, and it was agreed to collaborate on the study of the local plastic behaviour of the hpdc Mg alloys using the instrumented microhardness tester available at The University of Sydney, and to incorporate the existing 3D files of the intermetallic microstructure into an FE code to model the material's plastic behaviour.
Highlights of results published:
section of hpdc AZ91 alloy.
Mg 9%massAl alloy, and (c) uneven grain microstructure near the casting’s surface.
Future Activity Plan
Experimental work/ analysis of data
Microstructural analysis:
Experimental work/ analysis of data
Microstructural analysis:
Modelling
The analytical modeling of the contributions to the strength arising from grain size, solid solution, and dispersion hardening as well as from the 3D network structure, will be carried as the necessary hard data becomes available. See also the joint project with The University of Sydney.
Personnel
An applicant from overseas (D. Nagarajan) has been offered a University of Queensland living allowance and fee waiving scholarship to work on our project. Mr Nagarajan is currently waiting for his paperwork to be completed. His PhD project will evolve around the solute effects on the Hall-Petch constants in Mg-Al alloys. These data are crucial for the development of a quantitative model.
Joint project with USyd (Project B1.4)
A major collaboration program has been put together with Dr Luming Shen et al. (Project B1.4, simulation and modeling). This joint effort has two main streams:
The analytical modeling of the contributions to the strength arising from grain size, solid solution, and dispersion hardening as well as from the 3D network structure, will be carried as the necessary hard data becomes available. See also the joint project with The University of Sydney.
Personnel
An applicant from overseas (D. Nagarajan) has been offered a University of Queensland living allowance and fee waiving scholarship to work on our project. Mr Nagarajan is currently waiting for his paperwork to be completed. His PhD project will evolve around the solute effects on the Hall-Petch constants in Mg-Al alloys. These data are crucial for the development of a quantitative model.
Joint project with USyd (Project B1.4)
A major collaboration program has been put together with Dr Luming Shen et al. (Project B1.4, simulation and modeling). This joint effort has two main streams: